increasing its drift speed until it collides with a positive ion of the metal. It loses its drift speed after collision but starts to accelerate and increases its drift speed again only to suffer a collision again and so on. On the average, therefore, electrons acquire only a drift speed. (c) Simple, because the electron number density is enormous, ~ m – .
(d) By no means. The drift velocity is superposed over the large random velocities of electrons. (e) In the absence of electric field, the paths are straight lines; in the presence of electric field, the paths are, in general, curved. .
. Mobility As we have seen, conductivity arises from mobile charge carriers. In metals, these mobile charge carriers are electrons; in an ionised gas, they are electrons and positive charged ions; in an electrolyte, these can be both positive and negative ions. An important quantity is the mobility µ defined as the magnitude of the drift velocity per unit electric field: | | E µ = v ( .
) The SI unit of mobility is m /Vs and is of the mobility in practical units (cm /Vs). Mobility is positive. From Eq. ( .